Two general categories of solar energy collectors are known; the flat plate collector and the concentrating collector. For end uses of heat that require a relatively low temperature, such as space heating and domestic hot water heating, the flat plate collector is preferred because of its relatively cheaper construction and because it utilizes both direct and scattered solar radiation.
The general configuration of a flat plate collector is a shallow box with transparent wall or surface for passage of solar radiation to interior of the box, where it may be absorbed by an absorber plate and then transferred to a fluid such as air or water by convection or conduction.
The transparent wall or surface may be composed of one or more sheets of glass or plastic. This increases the efficiency of the collector by reducing convective and conductive heat losses as well as reducing radiative heat losses. This is true because whereas they are transparent to visible light, glass and, to a lesser extent, plastic are considered to be opaque to the longer wavelengths re-emitted by an absorber plate or the like within the box.
One of the problems in designing the absorber for a flat plate collector is to find a material that will absorb a large proportion of the energy contained in the incident light, efficiently transfer that energy to the fluid, and remain low in cost. A desirable collector is one with a high absorption coefficient, direct contact between the absorber and the fluid, turbulent flow of the fluid over the absorber, and a large contact surface between the absorber and the fluid.
Absorbers have been designed which include a black flat or corrugated surface, a flat plate with heat conducting fins, a flat plate with cup-like objects attached, or layers of blackened cloth.